Women who smoke for many years may increase their risk of developing breast cancer. New research shows that for women who had smoked for 40 years or longer, the risk of breast cancer was 60% higher than that of women who had never smoked.

Among those who smoked 20 cigarettes or more a day for 40 years, the increased risk rose to 83%. "Our findings suggest that smoking of very long duration and high intensity may be associated with increased risk of breast cancer," said the researchers, who say that few studies have looked at whether risk of breast cancer is associated with long term smoking. The researchers, from Albert Einstein College of Medicine, Bronx, New York, examined the association between cigarette smoking and incidence of breast cancer in a cohort of women who had smoked for up to 40 years at recruitment in the early 1980s; the women were subsequently followed for an average of 10.6 years (International Journal of Cancer 2002:100:723-8). The research, using data from almost 90 000 women in the Canadian national breast screening study, found that smoking intensity, smoking duration, years since smoking started, and pack years of cigarette consumption had positive associations with breast cancer risk. But age at which smoking began and years since quitting among former smokers were not clearly associated with risk. The researchers said that the positive association between smoking and breast cancer risk was driven largely by women who had smoked for 40 years, especially those who had smoked 20 cigarettes a day or more. The study found that women who had smoked either at least one pack of cigarettes a day for 40 years or at least two packs a day for 20 years were at noticeably higher risk than women who accrued the same number of pack years over a shorter duration. It adds, "Our findings, with respect to risk in association with duration of smoking years since smoking commencement and years since quitting, suggest that smoking may act primarily as an initiator rather than as a promoter of breast cancer, as has been hypothesised with respect to colorectal cancer." The researchers say that tobacco smoke contains many potentially harmful substances, including nitrogen oxides, volatile aldehydes, alkenes, and aromatic hydrocarbons, which may act differently and at different stages in the development of breast cancer. They add, "Since prospective cohort studies that have examined breast cancer incidence in relation to smoking duration of 30-40 years or more are scarce, confirmatory data are needed."

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